Food items such as meats, produce, and a variety of other products, are often packaged in see-through plastic wraps and bags made of polyolefin film. Such see-through plastic films enable customers to see the contents of the package clearly. However, condensation of moisture on unmodified polyolefin film can cloud or fog the package reducing the visibility. Therefore, films having anti-fog properties are desirable. Anti-fog properties relate to the ability of the film surface to prevent or to dissipate the condensation of water vapor into small, discrete droplets on the surface of the film.
The food packaging industry uses surfactant additives to confer anti-fog properties on polyolefin film surfaces. The surfactants are added in sufficient quantity to make the polyolefin surfaces, which are normally hydrophobic, wettable. If water vapor condenses on the surface of the film, it tends to form a thin, transparent layer instead of the discrete, cloudy-looking droplets.
One way to incorporate additives, such an antistats and anti-fog agents, into polymer films is to co-extrude the additive with the polymer, see J. P. Trotoir; Mod. Plast. 65(10) 102, 105-6 (1988).
A variety of additives for use with various film-making materials are known. See, for example, U.S. Pat. No. 2,561,010, which discloses the dispersion of various surfactants, such as hydrophilic fatty acid esters and polyoxyethylene ethers, in hydrophobic polymers used to make films, such as polyvinyl chloride, polyethylene, and cellulose acetate. U.S. Pat. No. 3,542,713 relates to the addition of polyoxyalkylene monoesterified with a fatty acid and a poly-alcohol esterified with a fatty acid to a poly(vinyl chloride) film.
It is known in the literature that certain akyl(polyether)alcohols are useful as anti-fog agents through their surfactant properties. Unfortunately, the use of these materials in such applications is limited by the problems that they cause during manufacture of the films. Their volatility which is low, is still sufficient to cause build-up around die lips of extruders for blown films. This build-up can cause imperfections in the film after extrusion or can even lead to shutdown of the process when the film bubble ruptures. PCT International Publication WO95/18845 describes the use of a composition comprising the following additives (i) a monoester of glycerol with a saturated or unsaturated fatty acid, (ii) a di-ester of a glycerol with a saturated or unsaturated fatty acid, and (iii) at least one ether of a polyoxyethylene with a fatty alcohol in films having anti-fog properties.
Polyoxyethylene (4) lauryl ether (HO(CH.sub.2 CH.sub.2 O).sub.4 (CH.sub.2).sub.11 CH.sub.3), a commercially available polyoxyethylene fatty alcohol, known as BRIJ.RTM., is similar to the compositions used in anti-fog applications. However, it undergoes a 17% weight loss at 200.degree. C. (64% at 300.degree. C.).
It is an object of the present invention to produce a polyolefin film which has latent anti-fog properties. It is an object of the present invention to provide a process for preparing such a film in a way that eliminates or decreases the amount of loss of an anti-fog agent during preparation. It is another object of the present invention to provide a process for preparing anti-fog film that reduces or eliminates the amount of build-up around the processing equipment, such as die lips of extruders, and the process problems evolving from such build-up, such as drips, and film bubble rupture.